Lens
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There are basically two types of lenses - convex
and concave. Convex lenses are thicker in the center and
thinner at the edges. Typical laboratory lenses are convex on both
sides, while some can be flat or even concave on one side as long
as the overall lens is thicker in the center.

Concave lenses are thinner in the center than at the edges. In
fact, a piece of plastic or glass that has parallel sides can be
thought of in one way as a "lens", even though it has no ability
to form images.